How Solar Hot Water Works

November 29th, 2011 by Fred Greenhalgh

Solar hot water systems work by capturing the thermal energy present in sunlight and turning it into usable energy in your home.

When solar hot water collectors (either flat plate or evacuated tube style) on your roof are warmer than your water tank, a differential temperature controller activates a pump. This pump then circulates a nontoxic antifreeze mix up to the solar collectors on your roof, where the fluid heats up.

This solar-heated fluid is then pumped to a super-insulated storage tank, where it gives off its heat as it passes through a heat exchanger. It is then pumped back to the roof, continuing the cycle as long as the sun is out.

Should there be unexpectedly high solar thermal gain (or should you be away), the system is designed to automatically go into an overheat protection mode called “Steamback.” Once a certain temperature threshold is hit, the solar circulating pump stops circulating fluid and the fluid present in the collectors turns to steam and fills an expansion tank. The copper elements of the solar hot water collectors are not harmed by high heat and will reach high temperatures until the end of the day. After the system has cooled overnight, the differential temperature controller will reset and allow the system to start running normally the following day.

From May to September, all of the hot water a household uses for washing dishes, clothes or taking showers will be provided by the sun, even on cloudy days. In the darker months of winter, the sun will still help out, but a backup unit – such as a boiler or electric element – will run to ensure a steady supply of hot water. Instead of running all summer, a home’s boiler will only fire up as a backup unit to heat the second coil in the upper part of your tank.

Overall a typical system will produce about 80% of a home’s water heating needs.


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